CenteredRectangleShape: exploit Tobias to get anisotropic rectangles

Could be merged with the RectangleShape.
Tobias?
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
Morphology: facilities to perform structuring elements decomposition

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>